This invention relates to the field of molded rubber articles. More particularly, this invention relates to the field of low sulfur-cured molded rubber articles and to methods of improving their fatigue life.
Natural and synthetic rubbers are compounded with various fillers, extenders and other ingredients and the resultant compound is formed under pressure into specific shapes after which it is cured or vulcanized under heat and pressure into a variety of articles (termed "vulcanizates"). Many of these vulcanizates are used in dynamic environments such as resilient (rubber) bushings that undergo the forces of compression, tension and torsion.
Most rubber articles are cured or vulcanized with a system that includes sulfur and an accelerator. The sulfur enters into the formation of crosslinks during curing. These crosslinks comprise two general types; monosulfide and disulfide crosslinks and polysulfide crosslinks. The literal differences between these are that the monosulfide and disulfide crosslinks consist of either one or two sulfur atoms connected between rubber hydrocarbon molecules; whereas a polysulfide linkage is any combination of three or more sulfur atoms joined together and connected between hydrocarbon rubber molecules. These two types of crosslinks impart various properties to the rubber matrix.
There are three properties of general interest with respect to the use of vulcanized rubber in resilient bushings. The first of these is reversion resistance. Reversion is the name given the depolymerization reaction that occurs in rubber both during and after vulcanization. During reversion the crosslinked rubber breaks down into smaller units and the physical properties of the vulcanizate suffer accordingly. Reversion is catalyzed by sulfur so that improved reversion resistance is associated with low-sulfur curing systems.
The second property is fatigue life or the ability of vulcanized rubber to withstand repeated flexing without significant failure of the crosslinked structure. This property is benefited by sulfur and is associated with high-sulfur curing systems.
The third property is compression set; this is the ability of the rubber to rebound to its original configuration after an applied force is removed from the article. Compression set is an inverse property of the sulfur content so that good (low) compression set is associated with low-sulfur curing systems.
These latter two properties are thought to be functions of the relative amount or density of mono- and disulfide linkages and polysulfide linkages. It is well-recognized that mono- and disulfide linkages are predominately produced in low-sulfur vulcanizing systems, i.e., less than about 1.5 phr sulfur, whereas in high-sulfur systems, i.e., about 2.0 phr sulfur and more, polysulfide linkages are produced in greater and greater amounts in lieu of the mono- and disulfide linkages.
One theory holds that mono- and disulfide linkages are relatively inflexible although strong and break down under repeated flexing whereas polysulfide linkages are more flexible although weaker but have the capability to reform to relieve the strain produced by the flexing. This would explain the enhancement of fatigue life in a higher sulfur content curing system. It would also explain why high sulfur content vulcanizates exhibit high (poor) compression set; the polysulfide linkages rearrange to relieve strain and thus have little driving force to further rearrange back to the original configuration when the forces are removed.
In certain rubber articles, such as in resilient bushings that comprise an annular rubber insert constrained under radial compression between inner and outer rigid sleeves, it is desired that the vulcanized rubber exhibit good reversion resistance and a good fatigue life. These properties ostensibly appear to be inapposite because, as explained above, reversion resistance is maximized at low sulfur levels whereas fatigue life is maximized at high sulfur levels.
Another aspect of these resilient bushings is that the time taken in vulcanizing the annular rubber insert is a significant cost factor. It is preferred to increase the vulcanization temperature to reduce the vulcanization time to reduce this cost. Such an increase in temperature would not only reduce vulcanization time but would allow the presently compression molded inserts to be injection molded and realize the additional cost savings associated with this high volume molding technique.
Unfortunately, increased vulcanization temperatures cause acceleration of the reversion reaction. At high levels of sulfur (to enhance fatigue life) the accelerated reaction may nullify all cost savings due to shorter vulcanization time whereas at low levels of sulfur (to enhance reversion resistance) fatigue life is seriously impaired.
This invention is based upon the discovery that a low sulfur-cured rubber article that has a normally high resistance to reversion, a low compression set, and poor fatigue life, may be treated subsequent to curing with additional elemental sulfur to significantly improve the fatigue life. Moreover, the sulfur treatment is confined to the surface of the vulcanizate so that other overall properties are not grossly effected.
Therefore, the main object of this invention is a method of treating a low-sulfur cured molded rubber article that normally has good compression set and good reversion resistance to generally improve its fatigue life. Other objects of this invention include a method of improving the fatigue life of low sulfur-cured rubber articles; a method of producing an improved molded part for use under dynamic conditions through the use of bi-level addition of sulfur, a process of potentially reducing manufacturing costs, a process that is simple and easy to practice and that is amenable to automatic and semiautomatic process control.